That is urates - completely normal. Tortoises do not create urea like humans do. Their metabolism (birds too) creates uric acid that is not as water soluable. This allow the uric acid to precipitate into semi-solid urates they can expell and not have to use precious water to "pee". The more water they have available, the more they will volutarily release their water and take in new water - lowering the uric acid concentration. So a well hydrated tortoise with good access to water will have less urates forming as the concentration is not has high. HIgher protein diets seem to allow for more urates as uric acid is a product of protein metabolism. I also believe lower body temperatures for the tortoise can cause more urates to form as uric acid's soluability is dramatically effected by temperatures.
